Xt discs brakes rubbing

uad782
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Xt discs brakes rubbing

Postby uad782 » Sun Aug 02, 2015 5:54 pm

Hi everyone,
I have just replaced my old rear disc brakes because they were always rubbing for the "cure all" of problems Shimano XT disc brakes and ice rotors.

Unfortunately this costly exercise has failed. The new XT brakes are always rubbing.

When I recentre the brakes by undoing the bracket and squeezing the lever I can get it pretty close but there is still some faint scraping. The scraping is not on the whole revolution of the rotor but only on a bit indicating that it may be slightly bent, however it is brand new.

Following a bit of riding the scraping gets noticably worse.

When I look at the gaps between the rotor and the pads it seems incredibly narrow.

So with my new brakes rubbing, I took the bike back to the LBS who advised this is normal mainly due to flexing in the frame. The bike is an aluminium dual suspension and has the old style hubs and QR skewers. They reckon bikes with the new through axles are much better and this wont happen. The bike is not a cheap dualie costing around $5k in 2011.

I mentioned that the gaps between rotor and pads is pretty tight and could they take out a bit of fluid, they said taking out fliud would result in air in the leads.
I don't believe that brakes should rub at all, even if it is just a slight scrape.

Can anyone please provide advice.

Thanks
Dave

eldavo
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Re: Xt discs brakes rubbing

Postby eldavo » Sun Aug 02, 2015 9:26 pm

No expert here, if it was a prob I would:
- Is the rotor flat, put on a flat surface for reference.
- With a square edge, without caliper, does it spin true with wheel mounted (any tolerance issue with wheel can cause eccentric, wheel bearings, axle?)
- are the pads genuine and free of manufacturer's faults? Read reviews of pads with variety of problems by cost cutting clones. Read a Ti Shimano pad delaminating.
- Shimano note 203mm are large and prone to rubbing more than smaller dia I assume.
- are calipers and seals good/clean for returning.
- Wonder what Zen and art of motorcycle maintenance says?

I get this is what you expect LBS to solve, maybe a 2nd LBS opinion?

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Duck!
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Re: Xt discs brakes rubbing

Postby Duck! » Sun Aug 02, 2015 10:07 pm

Rubbing on only one sector of the rotor loudly screams bent disc. Spin the wheel & sight through the caliper to see if there's a woof in the rotor. Shining a light through or holding a sheet of paper on the opposite side of your sight line to give a pale background will help with sighting the gap. Ice Tech rotors are good, but they're not without fault; the aluminium core layer is softer than a solid steel rotor, so is more prone to bending, and is a lot harder to straighten.

Make sure your skewer is done up firmly. Properly secured decent quality skewers are just as rigid as through-axles, despite what the mucketing people will tell you, but they are much more subject to variation in clamping pressure.

Despite the theory of opposed pistons suggesting that loosening the mount bolts & squeezing the brake on centring the caliper over the rotor, I find that for some reason it does not actually work most of the time. So using your backlight or pale background, manually centre the brake over the rotor with an even gap each side.
I had a thought, but it got run over as it crossed my mind.

uad782
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Re: Xt discs brakes rubbing

Postby uad782 » Mon Aug 03, 2015 7:23 am

Thanks. I note the park tools website says you cannot get rid of rubbing all the time. I will let you know how I go.

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Re: Xt discs brakes rubbing

Postby mitzikatzi » Tue Aug 04, 2015 2:22 pm

I do a bit of a combination of the squeeze method and by eye method.
A lazy piston can cause issues. A spray with brake cleaner and or some lubrication on the brake pistons. When I have had problems it has been solved by cleaning and lubricating the caliper pistons. Once the disc needed to be trued a little, by eye and just by thumb pressure.
The Hope brake website has a good video to do it by "eye"
I have also used feeler gauges for spark plugs. http://www.jensonusa.com/!44-QrifjdIEHy ... oCGgXw_wcB


http://www.hopetech.com/how-to-videos/#

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Re: Xt discs brakes rubbing

Postby Calvin27 » Tue Aug 04, 2015 3:11 pm

The chainstays do flex a lot. My rod bike with bb7 and a lot of clearance rubs frequently when I am putting down power. Sometimes if I am really giving it a lot of power (relative to me haha) the front will rub also. My mtb with thru axles deosn't have this problem whatsoever.

If you taped a camera to the seat stay pointing down at the caliper with a light pointing up and then go for a ride you'd be surprised how much the rear end actually moves. It's probably worse with a dually that is qr.

On a side note, I've found it's pretty unusual to have rotors come out of the box true. I'll get perfectly flat maybe 1 out of 3 rotors.
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Re: Xt discs brakes rubbing

Postby Duck! » Tue Aug 04, 2015 9:24 pm

Calvin27 wrote:
If you taped a camera to the seat stay pointing down at the caliper with a light pointing up and then go for a ride you'd be surprised how much the rear end actually moves. It's probably worse with a dually that is qr.

I suspect you'd actually find the opposite to be true; given that the suspension linkages would allow more movement than a rigid frame, less of the stress is going through the stays themselves, so that rear section of the frame would be less likely to flex, as it's moving as a whole against the main frame.
I had a thought, but it got run over as it crossed my mind.

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Re: Xt discs brakes rubbing

Postby zero » Wed Aug 05, 2015 3:16 am

uad782 wrote:
I mentioned that the gaps between rotor and pads is pretty tight and could they take out a bit of fluid, they said taking out fliud would result in air in the leads.
I don't believe that brakes should rub at all, even if it is just a slight scrape.

Can anyone please provide advice.

Thanks
Dave


My 9 year old deore brakes don't currently rub at all, at either end - they are currently on an alloy 2009 trek fuel dually frame. Even if the frame flexed, its going to return to the same position when its done flexing, and people notice disc rub when it won't roll without noise.

The gaps are extremely tight, most of the point of disc brakes is that they are a precision method of placing the pads close to the braking surface, so that the system can offer a lot of leverage. (leverage is a direct trade off against the distance the pads have to move).

You can't take fluid out in any case, its not how they work - either the reservoir fluid level is covering the lever valve and there is no air in the line and it works, or there isn't fluid covering the lever valve, air goes into the line and then at best it goes spongy and at worst it doesn't work at all.

The pistons actually sit in circular seals, as the pad wears more the lever is able to push the pistons further, so they take up new positions in the seals - and then the lever draws more fluid into the line through its valve. ie the seals don't hold the pistons in perfectly the same place, but oddly enough the seals also don't necessarily sit perfectly the same on the piston when the piston is in the same place, which affects how much/far the piston wants to spring back. Also if you push the pistons all the way back you get fluid on the piston sides, which then gets pulled onto the seal surface when you pump the lever after you've put it all back together again, and that may also help let the pistons move back more freely. Not guaranteed, but pushing the pistons back has certainly worked for me.

Also on my particular brakes, its possible to fit the retraction springs (you get a new spring with every pad set), incorrectly. there are little legs that are supposed to straddle the pads one particular way, and it is very hard to get them to go into the right spot. I'd imagine new brakes are less fussy than mine, but if they have external retraction springs, I'd check they are installed properly).

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Re: Xt discs brakes rubbing

Postby Calvin27 » Wed Aug 05, 2015 9:21 am

Duck! wrote:
Calvin27 wrote:
If you taped a camera to the seat stay pointing down at the caliper with a light pointing up and then go for a ride you'd be surprised how much the rear end actually moves. It's probably worse with a dually that is qr.

I suspect you'd actually find the opposite to be true; given that the suspension linkages would allow more movement than a rigid frame, less of the stress is going through the stays themselves, so that rear section of the frame would be less likely to flex, as it's moving as a whole against the main frame.


I was not thinking in the up/dwon direction. I think having more linkages and bearings will make it more flexy side to side, or torsionally. This is only opinion though, as I said, I noticed more movement in my road bike than my dually. But it's QR vs TA, so I can't really compare against rear end type.
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Duck!
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Re: Xt discs brakes rubbing

Postby Duck! » Wed Aug 05, 2015 9:57 am

Calvin27 wrote:
Duck! wrote:
Calvin27 wrote:
If you taped a camera to the seat stay pointing down at the caliper with a light pointing up and then go for a ride you'd be surprised how much the rear end actually moves. It's probably worse with a dually that is qr.

I suspect you'd actually find the opposite to be true; given that the suspension linkages would allow more movement than a rigid frame, less of the stress is going through the stays themselves, so that rear section of the frame would be less likely to flex, as it's moving as a whole against the main frame.


I was not thinking in the up/dwon direction. I think having more linkages and bearings will make it more flexy side to side, or torsionally. This is only opinion though, as I said, I noticed more movement in my road bike than my dually. But it's QR vs TA, so I can't really compare against rear end type.

Neither was I, and you've rewritten exactly what I said. The linkages allow the frame to "hinge" laterally rather than bend along the tubes, so the rear triangle in particular is more rigid than a hardtail frame.
I had a thought, but it got run over as it crossed my mind.

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Re: Xt discs brakes rubbing

Postby Calvin27 » Wed Aug 05, 2015 12:00 pm

Yeah got it.
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Magnum9
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Re: Xt discs brakes rubbing

Postby Magnum9 » Thu Feb 11, 2016 4:47 pm

Try adjusting the bite point adjusting screw. This may give you more clearance on the rotor if it is set up incorrectly.

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Bentnose
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Re: Xt discs brakes rubbing

Postby Bentnose » Sun Feb 14, 2016 4:19 pm

Have you tried pushing the pads out more and then rebleeding? There is a tool that comes with the brakes that enables you to push the pads out, it and the rebleeding may reset their position. I have SLX brakes of last years model or so and found them easy to bleed and set up, I even wrote a procedure based on the numerous different techniques people use on the net; some people make the bleeding process overly complicated. Have you tried adjusting them by hand/eye? The loosen, squeeze and retighten method doesn't always work.
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